Although this invention will be described with reference to use in haemodialysis, it will be appreciated that the various forms of the invention can be used wherever dual flow is required.
Haemodialysis can be defined as the temporary removal of blood from a patient for the purpose of extracting or separating toxins from the blood and returning the cleansed blood to the same patient. Haemodialysis is indicated in patients where renal impairment or failure exists, that is, in cases where the blood is not being cleansed naturally by the kidneys.
In the case of chronic renal impairment or failure, haemodialysis is carried out on a repetitive basis. For example, in end stage kidney disease where transplantation of kidneys is not possible or for medical reasons is contra-indicated, the patient will have to be dialysed about 100 to 150 times per year. This can result in several accesses to the bloodstream to enable the act of haemodialysis to be performed over the remaining life of the patient. The fact that dual flow is required to conduct haemodialysis means that there must be two distinct channels, one to remove the blood from the patient, and the other to return it. This was achieved in one approach by two insertions, each insertion carrying a single lumen catheter. Subsequently, dual lumen catheters have been inserted both by surgical cut-down techniques and also by engagement over a Seldinger wire using a technique developed by Dr. S. I. Seldinger which was presented at the Congress of the Northern Association of Medical Radiology at Helsinki in June of 1952. The technique remains current and is used widely.
It is clear that if a dual lumen catheter is to be inserted over a wire, the leading end of the catheter must be arranged to permit this engagement through tissue without tearing or snagging the tissue. An earlier approach to solving this problem was to make the dual lumen catheter of a co-axial construction which allowed the tip to be tapered for engagement through the tissue over the existing wire. Other catheters were developed where the lumens are arranged in side-by-side configuration and a tip formed especially to close off one lumen at a point spaced from the tip so that a tip could be formed around the return lumen to facilitate engagement over the wire. Structures of this kind can be found in U.S. Pat. Nos. 4,619,643, 4,583,968, 4,568,329, 4,543,087, 4,692,141, and 4,568,329. One of the disadvantages of this arrangement is that the structures result in stiff tips which although facilitating dilation of body tissue as the catheter is moved over the wire, they tend to result in relatively stiff structures inside the blood vessel after placement. As a result such catheters are useful only for temporary access.
If a catheter is to be used for extended placement, it must be extremely flexible to avoid stress in the blood vessel, and as much as possible, permit the catheter to move in the blood flow to minimize the possibility of the catheter remaining in pressure contact with the wall of the blood vessel at one spot for prolonged periods. It is also true, that if a catheter is designed for prolonged placement, then the very flexibility that is desirable for prolonged placement creates limitations for engagement over the Seldinger wire because the catheter lacks sufficient strength to dilate the tissue.
In summary, although there have been significant developments in the structures of dual lumen catheters with the lumens arranged in side-by-side configuration, these structures have been limited in their usefulness primarily because of the difficulties of meeting both design criteria required for placement by the Seldinger technique and the somewhat conflicting criteria which must be met for prolonged placement.
A further consideration in the design of dual lumen catheters is the positioning of the intake and return openings. In catheters of the type where the tip has been formed to dilate tissue as the catheter slides over a Seldinger wire, the intake openings are generally on the side of the catheter. This can result in the catheter being drawn by suction forces towards the blood vessel wall and blood flow will then be cut off. It is therefore desirable to arrange the intake opening to be at the end of the intake lumen with the opening extending generally transversely with respect to the longitudinal extent of the catheter. It is very unlikely that the blood vessel will occlude such an opening so that there is a better likelihood of continuous intake flow. On the other hand, this results in a catheter contour which is less than desirable for sliding over a Seldinger wire.
It is one of the objects of the present invention to provide a catheter having side-by-side dual lumens with the intake opening arranged generally transversely with respect to the longitudinal extent of the catheter and which can be engaged over a Seldinger wire.
It is a further object of the invention to provide a catheter for prolonged placement which has the necessary flexibility characterists and which can be engaged over a Seldinger wire.